use shortcut manager for the main help menu * main_window.cc (add_action): new parameter for the signal receiver; (construct_help_menu): use add_action for the actions of the menu; (construct_documentation_menu): use add_action for the actions of the menu; (set_global_shortcuts): set/unset shortcuts * main-window.h: actions are class variables, add_action with new parameter * shortcut-manager.cc (do_init_data): initialize the shortcuts in the help menu